The purpose of the proposed project is the study of hemostatic mechanisms and their relationship to other defenses of the body. In the coming year, particular emphasis will be upon surface-mediated reactions encompassing the interactions of Hageman factor, Fletcher factor (a plasma prekallikrein), Fitzgerald factor (high molecular weight kininogen) and plasma thrombo-plastin antecedent; activation of plasminogen; and the reported activation of Hageman factor by ADP-treated platelets. Additionally, studies of the nature of antihemophilic factor will be continued. BIBLIOGRAPHIC REFERENCES: Ratnoff, O.D., and Saito, H. A new function of Fitzgerald factor: evidence that Fitzgerald factor counteracts inhibition by kaolin or ellagic acid of the amidolytic properties of a plasma kallikrein. Blood 47: 243, 1976. Ratnoff, O.D., and Slover, C. Immunologic evidence that the properties of human antihemophilic factor (factor VIII) are attributes of a single molecular species. Blood 47: 657, 1976.